This invention is directed generally toward methods to synthesize crystalline films, superlattices and multilayered devices based on metallic or semiconductor compounds or alloys, in electrolyte media. More specifically, this invention is directed toward a method for the electrochemical synthesis of molecular layers of compounds or alloys on non-crystalline substrates.
Epitaxial semiconductor films and superlattices are currently sought for a wide range of device applications. Superlattice films can be designed with specific physical, electronic or optical properties. Material utilization, cost and environmental impacts are important concerns during superlattice manufacture. Presently such materials are synthesized with expensive, high temperature vacuum methods, molecular beam epitaxy (MBE) and atomic layer epitaxy (ALE). ALE and MBE methods allow the fabrication of advanced material structures of very thin oriented layers of metallic alloys or semiconductor films but economics and crystalline substrate requirements limit their use as research tools or for small, highly sophisticated expensive devices such as, quantum-well lasers.
Chemical bath deposition (CBD) has been employed for low cost, large area applications; it produces low quality semiconductor films. Electrodeposition offers a simple, low-cost and large area alternative to the vacuum or CBD techniques, in terms of the required capital equipment, power needs and material. It eliminates environmental and safety hazards associated with toxic vapor phase reactants and large volumes of chemical waste. These advantages have been explored by numerous researchers with limited success. Kroger et al first reported the electrodeposition of semiconductor compounds. U.S. Pat. No. 4,426,194 describes the approach. Unfortunately stringent material quality requirements for opto-electronic devices have excluded electrodeposition as an acceptable method for synthesis of semiconductor compounds. Recent works use electrodeposition to produce mainly precursor films as described in U.S. Pat. No. 5,730,832. These films require further vacuum processing to improve stoichiometry and grain size. Thus, economic and scale up barriers of the vacuum steps remain.
Conventional electrodeposition tends to produce small grains and non-stoichiometric films that are unsuitable for devices. The deposition is controlled by the mass transport rate of at least one metal, co-depositing to form a compound. This causes three-dimensional nucleation, hence small grain films. U.S. Pat. No. 5,320,736 describes an electrochemical method for atomic layer epitaxy for the deposition of semiconductors, comprising alternating electrodeposition of atomic layers of selected pairs of elements using underpotential deposition. This method eliminates the mass transport dependence and can produce atomically ordered layer of a compound. It uses a separate solution to deposit each of the component elements, constituting the compound. The use of two or more electrolytes to synthesize one compound evidently requires an elaborate deposition apparatus, a rinse cycle between deposition of each fraction of the monolayer, large quantities of electrolyte and very long (several hours/.mu.m) deposition period time. Thus, the method is impractical for manufacturing large devices, for example photovoltaic modules; such devices need low-cost, large-area, high-throughput deposition. For many compounds, this approach leads to the re-dissolution of one metal during deposition of the next. Nevertheless, the method provides evidence for the electrochemical epitaxial semiconductor formation.